A device or system that encrypts digital video content such as digital television has become to be used for the sake of copyright protection. Highly confidential information or secret data such as an encryption key required for the encryption of content is stored in a memory included in a semiconductor integrated (LSI) circuit and hence there is an increasing need to improve security strength for semiconductor devices.
Patent Literature 1 discloses an arrangement to improve security against an attack from a malicious user without deteriorating convenience for a legitimate user, by increasing delay time that delays a response to a key operation in accordance with the number of times that password authentication has failed. A summary of Patent Literature 1 is described below (refer to Patent Literature 1 for details).
A delay time D, that is a time from a key operation on an operation part until the generation of a response, is computed according to the following expression.D=S·log2(C+1)   (1)
Here, the parameter S is a coefficient that can be arbitrarily set in advance by a legitimate user, and is referred to as a security level S. The parameter C is the number of times that authentication has failed.
The delay time D of a response to a key operation increases logarithmically in accordance with the number C of authentication failures. Furthermore, the delay time D increases in accordance with the security level S set in advance by the user. The security level S can be arbitrarily set by the user, for example, to an integer from 1 to 10. The higher the security level, the longer the delay time D.    [Patent Literature 1] JP Patent Kokai Publication No. JP-P2009-258840A